slang is part of the language and I don't think that it should be looked from the value point. It happens that it is used by different groups. Therefore it is valid to that particular group as means of communication. As long as the memebers of the group communicate, the language, may it be slang or not, is a valid way of communication. Language usued by so called educated people is just a different form of language, no more valid, but valid to the educated people.